Adhering coatings to cathode base metal



July 15, 1958 D. R. KERSTETTER ET AL 2,843,517

ADHERING COATINGS TO CATHODE BASE METAL Filed March 24, 1955 Alkaline earth metal composition INVENTORS DONALD R. KERSTETTER ERNEST S. WENNIN ATTORNEY Unite States Patent -fiice 2,843,517 Patented July 15, 1958 ADHERING COATINGS T CATHODE BASE METAL Donald R. Kerstetter and Ernest S. Wennin, Emporium,

Pa., assignors to Sylvania Electric Products Inc., a corporation of Massachusetts Application March 24, 1955, Serial No. 496,608

8 Claims. (Cl. 154-429) In the prior art, a method of applying alkaline earth metal compositions held in a lacquer binder to metal cathode bases has been evolved in which the leaves are applied to the bases by wetting the bases with a lacquer solvent such as pentacetate and allowing, for each base, a leaf to come into adherence therewith or to wrap itself around the base element after initial contact with the base.

In the process of coating cathodes with emitter coatings of a mixture. of barium, strontium and calcium carbonates by the wrap-around method described above, difliculties have been experienced in that the coating peeled or separated from the cathode base metal, usually a nickel alloy with a high percentage of nickel. This separating occurred when the cathode was subjected to heat in a vacuum during the process of reducing the carbonates to oxides. This is not only a problem peculiar to wrap-around coatings but is frequently experienced with coatings applied by spray or cataphoresis coating techniques. A number of changes in processing can be used to correct this problem such as special cleaning of the cathode base metal, firing the base metal, roughening it or changing lacquers or solvents in the coating. However, none of these is completely effective.

It is an object of this invention to provide a simple method of applying the carbonates to the metal base of the cathode so as to almost completely avoid loose coatings.

The figure illustrates in cross-section a cathode made in accordance with the invention.

We have found that the application of a very small amount of barium nitrate between the cathode base metal and the cathode coating has resulted in coatings which adhered so firmly to the cathode base metal as to reduce the loss due to flaking off or peeling of the coating to a negligible amount. This is apparently brought about by one or a combination of two characteristics of the barium nitrate. This compound fuses at the relatively low temperature of 592 C. which is a temperature between the burn-ofi temperature of the lacquer binders normally used to hold the carbonates together and the sintering or fusing temperature of the oxides. The barium nitrate is held in suspension in a liquid which readily wets the cathode base metal. Such a liquid may be the pentacetate referred to above. As a result, the barium nitrate suspension readily covers the surface of the metal. The fusion of the barium nitrate causes it to act as an adhesive between the coating and the cathode base metal thus insuring good bonding and freedom from peel between the metal oxides and the base metal, after reaching the temperature at which the metal oxides fuse. A second action which takes place is that on heating in a vacuum, the barium nitrate, Ba(NO breaks down, in part, into oxides of nitrogen which have an oxidizing effect on the metallic cathode base. This oxidation of the cathode base metal assists in good bonding between the emissive oxides of the coating and the cathode base metal because of the fact that there is good adhesion between the nickel and nickeloxide interface of the cathode base and far better ad. hesion between the nickel oxide and the combination of the earth metal oxides.

It is important that the barium nitrate be applied directly to the cathode base metal. Although adding it to an earth metal carbonate strip as an ingredient of the strip and prior to Wetting a leaf from that strip with pentacetate has some beneficial effects, it is most eifective when applied in such a manner as to be in direct contact with the base metal and interposed between the metaland the leaf.

This may be accomplished in one of several ways. The cathode base may be sprayed with a suspension of barium nitrate in pentacetate, a concentration of .05 to .15 barium nitrate in the pentacetate working very satisfactorily. After the cathode base has been coated in this manner, it can be further coated by the normal spray or cataphoresis technique or with the wrap-around coating as referred to above. It has been found advisable to fire the cathode surface when the barium nitrate is on it so that the barium salt will fuse to the cathode base and will not in, or prior to, subsequent operations be readily washed or blown off. In the case of the wrap.- around coating, it has been found that the barium nitrate may be applied to the coating very simply by adding the barium nitrate to the pentacetate that is being used to eifect self-wrapping of the coating around the cathode base. Thus the cathode base is wetted with a solution of pentacetate containing the barium nitrate and then the leaf for film of cathode coating is brought into contact with the wetted cathode base to allow the wrapping action to take place.

Still another method may be used to apply the barium nitrate. This can be accomplished during the preparation of coating film by first laying down on a smooth surface a thin film of barium nitrate from a volatile suspension with some lacquer binder and, after the solvent evaporates, laying the carbonate coating in a similar volatiie suspension lacquer binder over the nitrate coating. The composite film, when dry, is then stripped off the surface and applied to the cathode base with the barium nitrate side of the film toward the base. This film will wrap itself around the base in the same manner as the carbonate coating alone would, on the wetting of the base with the solvent for the binder in the barium nitrate or in the emitter coating.

The quantity of barium nitrate necessary to produce satisfactory results is extremely smallsomething in the order of .1 mg. to 1.0 mg. of barium nitrate per square centimeter of coating.

Having thus described the invention, what we claim is:

1. In the manufacture of a cathode for use in electron tubes and which cathode includes a metal base and a preformed layer of alkaline earth metal compounds, the step of interposing barium nitrate between the metal base and the layer of metal compounds.

2. In the manufacture of a cathode for use in electron tubes and which cathode includes a metal base containing a nickel alloy and a preformed layer of alkaline earth metals comprising the carbonates of barium, strontium and calcium, the step of interposing barium nitrate between the metal base and the carbonate layer.

3. In the manufacture of a cathode for use in electron tubes and which cathode includes a base of nickel alloy, the step of applying a coating of barium nitrate to the base and subsequently applying a preformed layer of an electron emissive coating containing the carbonates of barium, strontium and calcium.

4. In the manufacture of a cathode for use in electron tubes and which cathode includes a base of nickel alloy, the steps of applying a coating of barium nitrate to the base, fusing the barium nitrate onto-the-base, applying a preformed layer of an electron emissive coating containing the carbonates of barium, strontium and calcium and beating: the assemblyto convertthe carbonates into oxides and to bond the base withtheoxides;

'5. Inthe manufacture of a cathode foruse in electron tubes and which cathode includesabase of nickelalloy and an overlyingfilm in a soluble binder which film contains the carbonates of barium, strontium and-calcium, the step of applying -tosaid base a solvent for the film binder which solvent contains a dispersion of barium nitrate, and, while-said-base is still wen-applying thereto. the carbonate film.

. 6. In the. manufacture of a cathode for use in electron tubes and which cathode includes a base-of-anickel alloy and a coating of carbonates of barium, strontium and calcium, the steps of preparing asuspension of barium nitrate in -a volatile liquid, pouring ,the same onto a smooth surface to form a smooth thin layer, preparing a second suspension .of the carbonates of barium, strontium and calcium in a volatile liquid with a binder dissolved therein, pouring said second suspension over the thin layer to form a composite film, and applying said composite film to the-alloy base, barium nitrate side toward the alloy, substantially simultaneously with-the addition of a solvent for 'the binder between the alloy base and the film.

-7. In the-manufacture of a cathode for use inielectron tubes and which cathode includes a metal base and a layer of alkaline earth metal compounds, the step of applying to the metal base both barium nitrate and the layer of metal compounds, with the barium nitrate in coexistence with the metalcompounds and interposed between the metal base and compounds.

8. In the manufacture of a cathode for use in electron tubes and which cathode includes a metal base containing a nickel alloy and alayer of alkaline earth metals comprising the carbonates of barium, strontium and calcium, the step of applying to the metal base both barium nitrate and the layer of carbonates, with the barium nitrate in coexistence with the earth metals and interposed between the metal base and earth metals.

References Cited in the file of this patent UNITED STATES PATENTS 1,812,103 MacRae June 30, 1931 2,238,595 McNall Aug. 1 15, 1941 2,650,884 Baksweret al. Sept. 1, 1953 2,744,838 Newman May.8, 1956 OTHER REFERENCES vMellofls Comprehensive Treatise on Inorganic and TheoreticallChemistry (1923), vol. 3, p. 849. 

1. IN THE MANUFACTURE OF A CATHODE FOR USE IN ELECTRON TUBES AND WHICH CATHODE INCLUDES A METAL BASE AND A PREFORMED LAYER OF ALKALINE EARTH METAL COMPOUNDS, THE STEP OF INTERPOSING BARIUM NITRATE BETWEEN THE METAL BASE AND THE LAYER OF METAL COMPOUNDS. 